Not Applicable
1. Technical Field of the Invention
The present invention relates generally to an alternate current to direct current and then back to alternate current (AC/DC/AC) power conversion system, and more particularly to an improved power structure coupled with a control means of an AC/DC/AC power conversion apparatus designed to convert fixed frequency AC source into variable frequency and variable voltage source for supplying power to an electric motor.
2. Background Arts
FIGS. 1 and 2 show power structures of conventional AC/DC/AC power converter for driving an AC motor. The converter typically includes a rectifier, a DC link and an inverter. Majority of ac drives do not need to provide regenerative feature and hence a diode rectifying front end is cost effective and suffices as the rectifier. The rectifier directs the AC power from the ac lines to provide power with a DC plus a six times line frequency component as input to the DC link capacitors. The inductor L on either the DC link or the input side is helpful in improving power factor of the power converter. The inverter modulates the DC voltage to generate variable frequency and variable voltage output to a motor.
The DC link capacitor section consists of at least two electrolytic capacitors C1 and C2 in series to have high enough voltage withstanding capability for the DC bus. Typical voltage rating for an electrolytic capacitor is about 450 VDC, not enough to work under a 650 VDC bus for an power converter. The series stack up of these electrolytic capacitors in turn requires the voltages across each capacitor to be equal and balanced. Such requirement is fulfilled by addition of R1 and R2 which may be made up by more than one single resistor to withstand enough watt loss. Resistors R1 and R2 supply a current leakage path to balance the capacitor voltages. R1 and R2 also serve to discharge C1 and C2 when the power converter is not in use. Capacitor C3 is either a type of film or ceramic capacitor whose capacitance is much smaller than provided by C1 and C2. C3 is primarily a snubber capacitor for mitigating any high voltage transient caused by high slope switching from within the inverter section.
There are drawbacks of conventional power converter using electrolytic capacitors:
A large capacitance electrolytic capacitor which must withstand a high level of DC voltage has a relatively limited operating lifetime. The effective operating lifetime of a power converter as whole will in general be determined by this smoothing electrolytic capacitor.
Breakdown of such an electrolytic capacitor can cause serious damage to other components of the power converter, since leakage of corrosive electrolyte may occur, or the capacitor may even explode.
Because of electrolytic characteristics, other components such as balancing resistors and high frequency film capacitor are required. These additional components increase not only cost but also board space to mount them.
The smoothing electrolytic capacitors occupy a relatively large amount of space within the power converter, thereby reducing the freedom available for mechanical design of the converter, and causing the overall size of the power converter to be large.
The power converter manufacturers and research community recognize above problem and realize improvement of AC drive product can be made if electrolytic capacitors are to be replaced. U.S. Pat. Nos. 5,729,450, 5,623,399, 6,115,270 and 6,449,181B1 propose to use film capacitor in place of electrolytic ones to provide ripple current capability to the inverter section. U.S. Pat. No. 5,623,399, U.S. Pat. No. 6,115,270 and U.S. Pat. No. 6,449,181B1 are invented for new electric vehicle applications where a battery is readily available to serve as a huge energy storage component in front of the inverter. The other patent U.S. Pat. No. 5,729,450 is in effect teaching for inverting energy from a fuel cell DC source to AC power. The fuel cell has low impedance and again serves as the huge energy storage and imposes smoothing effort on the DC link. Some of above patents, for example U.S. Pat. No. 5,729,450 claims that their approach also applies to conventional AC power converter product with a non-regenerative front end such as diode bridge. We would like to dispute here. If we assume that the energy storage component (in this case battery or fuel cell in stead of electrolytic capacitors) is taken away from the circuit, there is little energy storage capability left by the film capacitor on the DC link. Conventional AC power converter has working condition where transient energy can come back from the motor to the DC link. In this case the DC link is easily over charged, resulting in DC link over-voltage. This over voltage may damage the converter components such as the switching transistors. Thus their solution is not readily applicable to a conventional AC power converter with a diode front end rectifier.
U.S. Pat. No. 5,481,451 also proposes film capacitor in place of the electrolytic capacitors however only focuses on how to re-shape the inverter output voltage and current to the motor since the DC bus voltage is no longer only a DC value. As a matter of fact, the patent does not indicate requirement of the kind of front end rectifier and for the same reason as discussed in previous paragraph, we know that the taught technique in U.S. Pat. No. 5,481,451 alone will not work for an AC power converter with diode front end. Additionally this technique limits the output voltage transfer ratio to 0.866 which is not tolerable to most of the AC power converter products.
When energy comes back from the AC motor, the DC link voltage can easily be over voltage if the AC drive employs non-regenerative frond end such as a diode bridge. Such over-voltage can be suppressed by a snubber circuit. U.S. Pat. Nos. 6,169,672B1, 5,561,596, 5,157,574, 4,843,533 and 4,646,222 propose this kind of circuits with substantially more components either on DC link, ac input or output to suppress the over voltage spike. There are cases where active switch is employed and further requires active control from a controller. Complexity and part counts are increased and hence also the cost.
One particular solution for the DC link over voltage problem is to replace the non-regenerative diode front end with a regenerative-capable rectifier. Kim et al, in a technical paper xe2x80x9cAC/AC Power Conversion Based on Matrix Converter Topology with Unidirectional Switchesxe2x80x9d published in IEEE Transaction on Industry Application of January/February 2001, pages 139-145, taught such solution. A regenerative capable rectifier usually consists of a boost type converter which utilizes active switches. Cost again is an issue for those applications that do not require regenerative feature.
U.S. Pat. No. 5,825,639 introduces a diode and a resistor to isolate the electrolytic and film capacitors. Such approach intends to let the film capacitor handle majority of the inverter ripple current and to reduce the stress to the electrolytic capacitors and their capacitance. However, a continuous loss through the additional resistor is present in order to render effectiveness from the electrolytic during transients. Besides the system suffers poor efficiency, additional diode and resistor also offset the cost saving from the reduction of electrolytic capacitance. Such arrangement is not able to deliver storage energy from the electrolytic capacitors during ride-through. The electrolytic capacitor, resistor and the diode taught by U.S. Pat. No. 5,825,639 is only a powerful over voltage clamp.
Part of above aforementioned inventions teaches inverter structure with reduced capacitance by addition of snubber circuit. The resultant power converter is costly, less efficient as well as bulky. Other part of above aforementioned inventions teaches reduced capacitance inverter to interconnect either to a large storage DC source or a regenerative rectifier. Both configurations results in either regulated or slow moving DC bus voltage. The speed to feedback this bus voltage is furnished by the controller of the power converter, and is not required as fast. When a non-regenerative rectifier is used in place of above large DC source or a regenerative rectifier and feeds input to this reduced capacitance inverter, the bus is neither regulated nor slow moving. DC bus voltage feedback quality by conventional controller deteriorates significantly during fast bus voltage transient.
Now it becomes apparent that none of above solutions is suitable or adequate to make a reduced capacitance DC/AC inverter capable of working with a non-regenerative diode rectifier. It is the objective of the present invention to overcome aforementioned drawbacks.
It is the objective of the present invention to construct AC/DC/AC power converter systems without using any electrolytic capacitor, such that the converter is more compact, durable and reliable. These converter systems only require a small capacitance easily obtainable with other types of capacitors such as film or ceramic type. The system further includes means to disconnect both input and output to the DC bus capacitor. A controller capable of fast monitoring the DC bus voltage is also part of the system. The controller is also able of quickly disconnecting the capacitor out of either input or output energy path to prevent the capacitor from being charged to over-voltage. The controller further possesses capability of re-connecting the disrupted energy path once the DC bus capacitor voltage returns to normal.